1. Field of the Invention
The present invention relates to a compact zoom lens which has a high variable magnification and can be used as a photographing lens for a 35 mm lens shutter camera.
2. Description of the Related Art
A general zoom lens used for a 35 mm lens shutter camera is basically of a telephotographic type to reduce an entire length of the zoom lens.
A simplest zoom lens of the telephotographic type is often constructed by a first lens group having a positive focal length and a second lens group having a negative focal length.
However, when the zoom lens of this type is used to obtain a high variable magnification, an F-number of the zoom lens at a telescopic end thereof is extremely increased and a moving amount of the second lens group having a negative focal length is extremely increased when a zooming operation is performed from a wide angle end of the zoom lens to the telescopic end thereof.
To solve these problems, for example, Japanese Patent Application Laying Open (KOKAI) No. 62-50718 and U.S. Pat. No. 4,828,372 show a lens structure constructed by a first lens group having a negative focal length and a second lens group having a positive lens group. This second lens group is constructed by a front lens group having a positive focal length and a rear lens group having a negative focal length. An entire length of the lens structure is reduced by such first and second lens groups.
However, in the lens structure shown in Japanese Patent Application Laying Open (KOKAI) No. 62-50718, the distance between principal points of the front and rear lens groups constituting the second lens group is considerably short in comparison with the focal length of the front lens group. Therefore, refracting power of the front lens group in the second lens group is increased so that it is difficult to reduce an F-number of the lens structure and increase brightness of the lens structure. Further, a back focus of the lens structure is also increased so that the entire length of the lens structure cannot be necessarily reduced sufficiently.
In the lens structure shown in the above U.S. Patent, the distance between principal points of the front and rear lens groups constituting the second lens group is set to be longer than the focal length of the front lens group. Therefore, an F-number of the lens structure is small and a back focus thereof is also short. However, a ratio of a maximum entire length of the lens structure to a focal length of the lens structure at a telescopic end thereof is equal to about 1.1 so that the maximum entire length cannot be necessarily reduced sufficiently.